Similar Genes, Different Personalities

SiblingsIllustration by Barry Falls

I had an NPR driveway moment earlier this week, listening to a piece by Alix Spiegel about why siblings are so different.

After all, she points out, you are far more likely to share physical characteristics with a sibling than you are with someone randomly chosen from the population. This is dramatically true of cognitive abilities as well.

But personality? Studies have found that when it comes to things like extroversion or conscientiousness, siblings are similar only 20 percent of the time, which is only the slightest bit higher than a comparison to a random stranger.

At first that sounds impossible. Wouldn’t people who share genes and parents and a childhood home be likely to turn out alike?

Then I thought of my own family. My brother and sister and I do look alike, and (though we would not have conceded this when we were much younger) we are all similar in intelligence. But in so many other ways we are opposites. Get the three of us together and at least someone in the room can be described as garrulous or taciturn, cheap or indulgent, meticulous or sloppy, dramatic or cool. A Venn diagram would have some overlap, but not much. How is this possible?

There are three theories, Spiegel reports. The first, known as “divergence,” postulates that children are competing for their parents’ “time, love and attention,” and, as Darwin pointed out, organisms tend to “diverge” and specialize to minimize direct competition. “So if one child in a family seems to excel at academics,” Spiegel says, “the other child — consciously or unconsciously — will specialize in a different area, like socializing.”

The second theory, that of a nonshared environment, argues that we only seem to be “growing up in the same family as our siblings,” when in fact we are being raised in measurably different households. We experience the same events at different ages; we have different numbers of older and younger siblings; we are born to different parents — older or younger, wealthier or struggling, happier or less so, more experienced, less energetic.

And, finally, there is the theory of “exaggeration.” It is similar in result to “divergence,” though different in its cause. A child who might be considered outgoing and social in a family of introverts, becomes the “quiet one” in a family of extroverts. She then thinks of herself that way, chooses quieter friends and gradually becomes somewhat quieter. (As parents we are supposed to make sure this doesn’t happen — that we don’t compare our children. That is, of course, impossible. But it’s the reason I’m not mentioning my own children in this post. Exaggeration and divergence may be the natural order of things, but declaring in print that one child is vanilla and the other is chocolate is still a bad idea.)

NPR is spending the week looking at siblings. Apt timing. As Spiegel said:

…as you eat your turkey, look across the table. There, you may see a brother, a sister, a step-sibling, a twin. And maybe they’re your friend, and maybe they’re your enemy, but one thing is for certain: Their very existence has had a profound influence on your life.